Fiber base material for wet friction material

ABSTRACT

A fibrous base material giving a wet frictional material used for clutch, differential gear, brake, etc. of cars, industrial machines, etc. having an excellent durability where plastic deformation of fiber and deterioration by acidic decomposition products of lubricant additives do not take place is provided. A fibrous base material for wet frictional materials which comprises a fiber component, a friction adjusting material and a filler, said fiber component containing 5 to 50% by weight of a homoacrylonitrile fiber where modulus of elasticity at 180° C. is not less than 10 cN/dtex and shrinking rate at 200° C. is within 5% and/or a homoacrylonitrile fiber in a pulp form prepared by fibrillation of said homoacrylonitrile fiber.

TECHNICAL FIELD

[0001] The present invention relates to a fibrous base material for wetfrictional materials which is a fibrous base material useful for theformation of wet frictional material used for clutch, differential gear,brake, etc. of automobiles, cars, industrial machines, etc. and isparticularly excellent for its durability.

BACKGROUND OF THE INVENTION

[0002] A wet frictional material which functions in lubricant havingboth effects of cooling and lubrication is used in automatictransmission for automobiles, differential gear for four-wheel-drivevehicles, wet brake of tightly closed type, etc. With regard to anorganic fiber material for the wet frictional material, there have beenused acrylic fiber and fibrillated acrylic fiber which are lessexpensive as compared with aramid pulp and have excellent adhesiveproperty to a bonding material where improvement in strength of the wetfrictional material can be attempted and an appropriate porosity isachieved. For example, in an invention of the Japanese patentpublication No. 05/45,808 B, there is proposed a wet frictional materialwhere fibrillated acrylic fiber in an amount of 5 to 50% byweight to thetotal fiber is used and, in the Japanese patent publication No.07/37,606 B, there is proposed a wet frictional material using a fiberbase material by the joint use of acrylic pulp and aramid pulp. However,as a result of an increase in generating power of car engine andtendency of making efficiency and size of automatic transmission higherand smaller in recent years, sliding speed increases and load uponengagement becomes high resulting in high temperature of the wetfrictional material whereupon durability of the wet frictional materialhas become a problem.

[0003] Thus, when acrylic fiber and fibrillated acrylic fiber which havebeen used as fibrous base materials for wet frictional materials aresubjected to stress such as sliding and compression for a long period ina wet frictional material of high temperature, there is a disadvantagethat elastic modulus extremely lowers and fiber is plastically deformed.As a result thereof, the wet frictional material is deteriorated, poresformed among the fibers are crushed and appropriate oil membrane is notprepared whereupon there is a problem that frictional characteristiclowers and that, finally, burning is resulted. Usually, a sulfurcompound such as calcium sulfonate or zinc dialkyldithiophosphate isadded to a lubricant as a cleansing dispersing agent and ananti-abrasion agent but such a compound produces acidic decompositionproducts by heat. They accelerate deterioration of not only jointly usedcellulose pulp but also acrylic fiber and fibrillated acrylic fiber usedconventionally for the wet frictional material whereupon durabilitylowers and that is another problem.

OBJECT OF THE INVENTION

[0004] An object of the present invention is to provide a fibrous basematerial giving a wet frictional material particularly used for clutch,differential gear, brake, etc. of cars, industrial machines, etc. havingan excellent durability where plastic deformation of fiber anddeterioration by acidic decomposition products of lubricant additives donot take place.

SUMMARY OF THE INVENTION

[0005] It has been found that, according to the present invention, theabove-mentioned problem can be solved by the use of a homoacrylonitrilefiber which keeps a certain modulus of elasticity in a frictionalmaterial of high temperature and has little thermal shrinkage. Thus, thepresent invention relates to a fibrous base material for wet frictionalmaterials which comprises a fiber component, a friction adjustingmaterial and a filler, said fiber component containing 5 to 50% byweight of ahomoacrylonitrile fiber where modulus of elasticity at 180°C. is not less than 10 cN/dtex and shrinking rate at 200° C. is within5% (hereinafter, may be referred to as “homoacrylic fiber”) and/or ahomoacrylonitrile fiber in a pulp form prepared by fibrillation of thehomoacrylonitrile fiber (hereinafter, may be referred to as “homoacrylicpulp”). Incidentally, in the fibrous base material of the presentinvention for wet frictional materials, ratio by weight of thehomoacrylic fiber to the homoacrylic pulp is from 50/50 to 10/90 and thehomoacrylic fiber is a fiber which is subjected to a crimping treatmentfollowed by cutting into 0.5 to 4 mm (hereinafter, may be referred to as“curved homoacrylic short fiber”) and, as a result thereof, an object ofthe invention is able to be more highly achieved.

DETAILED DESCRIPTION OF THE INVENTION

[0006] As hereunder, the present invention will be illustrated indetail. A polymer which is a starting material for the homoacrylic fiberand/or the homoacrylic pulp adopted by the present invention is ahomopolymer in which only acrylonitrile monomer is used as a type of apolymerizing monomer. In the so-called acrylic fiber using another typeof monomer as a copolymerizing monomer (hereinafter, referred to as“copolymer acrylic fiber”), crystallinity of the copolymer is disorderedand, even when any spinning means for improving the crystallinity asmentioned hereinafter is adopted, degree of crystallization is notimproved whereby, even when it is adopted as a fibrous base materialcomponent for wet frictional material, modulus of elasticity is unableto be maintained at the state of high temperature resulting in a plasticdeformation. In a wet frictional material where such a copolymer acrylicfiber is used as a fibrous base material for wet frictional materials,the fiber is deteriorated when used for a long period and, as a result,there is generated a problem that coefficient of friction is unable tobe maintained. In addition, in the copolymer acrylic fiber, resistanceto acidic decomposition products resulted by additives in the lubricantis low and there is a problem that, when exposed thereto for a longperiod, deterioration happens and strength of the fibrous base materiallowers whereby an object of the present invention is unable to beachieved. On the other hand, in the case of the homoacrylic fiber whichis a homopolymer of acrylonitrile adopted in the present invention,resistance to chemicals such as resistance to acid is very high and allof the problems as a result of adoption of the copolymer acrylic fibercan be solved.

[0007] With regard to a solvent for spinning a polymer which is a rawmaterial for the homoacrylic fiber and/or the homoacrylic pulp, knownones may be used and, when spinning is carried out by an organic solventrepresented by DMSO, DMA and DMF, there is a tendency that modulus ofelasticity at high temperature and resistance to acid somewhat lower.Accordingly, an inorganic aqueous solvent such as aqueous solution ofsodium thiocyanate (hereinafter, referred to as PRS), nitric acid or anaqueous solution of zinc chloride is particularly preferred.

[0008] In the homoacrylic fiber and/or the homoacrylic pulp, modulus ofelasticity at 180° C. is not less than 10 cN/dtex or, preferably, notless than 13 cN/dtex. Here, the modulus of elasticity at 180° C. meansthat measurement of modulus of elasticity according to JIS-L-1095 iscarried out in a dry and hot box of 180° C. and an inclination at thestage where tensile strength of the fiber is 5% in a chart of therecorded load/ductility is calculated according to the above JIS.

[0009] Although temperature of a circulating lubricant having a functionof cooling a wet frictional material is about 120° C., temperature of awet frictional material rises up to about 180° C. due to high efficiencyand high mechanism of the recent automatic transmissions. When modulusof elasticity of the homoacrylic fiber and/or the homoacrylic pulp isless than 10 cN/dtex, pores are crushed, stable frictional coefficientis unable to be maintained and, in the worst case, burning happens whenthe wet frictional material is operated for a long period whereby anobject of the present invention cannot be achieved.

[0010] In order to achieve the modulus of elasticity at 180° C. for thehomoacrylic fiber which is an essential feature of the presentinvention, it is necessary to rise the degree of crystallization of thefiber. For rising the degree of crystallization of the fiber, it isessential that the polymer used as a starting material is a homopolymerof acrylonitrile and known techniques such as high polymerization ofpolymer, high concentration of spinning polymer dope, spinning by airgap, dry-heat high drawing after spinning, etc. may be appropriatelycombined and adopted and, in order to obtain a preferred modulus ofelasticity at 180° C., it is necessary to adopt all of such techniques.

[0011] Incidentally, there is no particular limitation for molecularweight, concentration of spinning polymer dope and dry-heat draw ratioafter spinning provided that the homoacrylic fiber having thecharacteristics of the present invention can be prepared and, withregard to the molecular weight, an example in terms of weight-averagemolecular weight (Mw) is 150,000 or more or, preferably, 200,000 ormore. Concentration of the spinning polymer dope may be decided bytaking molecular weight, type of the solvent, viscosity of the dope,operation stability in the spinning step, etc. into consideration. Forthe dry-heat draw ratio after spinning, it is also necessary to takedry-heat temperature and operation stability into consideration and,although it cannot be simply decided, an example is 1.6 times or moreor, preferably, 2.0 times or more.

[0012] It is also necessary that shrinking rate at 200° C. of thehomoacrylic fiber and/or the homoacrylic pulp in the present inventionis within 5%. For the production of a wet frictional material, a heatingtreatment at about 200° C. with pressure is applied during athermosetting process of the fibrous base material of the presentinvention with a binder such as a thermosetting resin and in a step ofcore metal adhering process in molding into the final shape and, in thehomoacrylic fiber of the present invention, no plastic deformationhappens even in a state of such a high temperature. However, whenshrinkage of the fiber at 200° C. is more than 5%, the fibrous basematerial shrinks during the heating process as such resulting in aproblem that the size of the wet frictional material does not becomeconstant whereby it is unable to be used as a fibrous base material forwet frictional materials.

[0013] Here, measurement of the shrinking rate at 200° C. is done asfollows. Thus, for 100 cm which is measured and marked under such astate that a load of 0.1 cN is applied per dtex of fiber flux at roomtemperature, the length which is measured in such a manner that thematerial is allowed to stand with a non-loaded state at 200° C. for 10minutes in a hot-air drier, subjected to natural cooling until roomtemperature and loaded again in the same manner is defined A (cm) andthen calculation is done by the following formula.

Shrinking rate (%)=100−A

[0014] With regard to a method for the preparation of fiber having anaimed shrinking rate while the above-mentioned modulus of elasticity at180° C. is still maintained at 10 cN/dtex or more, an example is amethod where the spun homoacrylic fiber is subjected to a heat settingtreatment for at least 1 minute at 205° C. or higher under the tensiletension where the size is maintained.

[0015] The homoacrylic fiber which is prepared as above may be cut intoan appropriate size and used as it is or, as will be mentioned later, itmay be used as a homoacrylic pulp. Alternatively, both may be combinedand used. The fibrous base material for wet frictional materialsaccording to the present invention comprises a fiber componentcontaining 5 to 50% by weight of the homoacrylic fiber and/or thehomoacrylic pulp as such, a friction adjusting material and a filler.

[0016] Here, the ratio of the homoacrylic fiber and/or the homoacrylicpulp to the total fiber components of the fibrous base material for wetfrictional materials is 5 to 50% by weight. When it is less than 5% byweight, the above-mentioned effect of the homoacrylic fiber and/or thehomoacrylic pulp is not significantly noted while, when it is more than50% by weight, balance to other fiber component is bad and there is apossibility of causing the deterioration of properties as frictionalmaterials whereby that cannot be adopted. With regard to other fibercomponent to be used for fibrous base material for wet frictionalmaterials, publicly known heat-resistant fiber and pulp may be utilizedand, in view of balance between heat resistance and cost, it is usual touse cellulose pulp such a linter pulp together with aramid pulp. Besidesthe above, it is also possible to use inorganic fiber having good heatresistance such as carbon fiber, glass fiber, potassium titanate fiber,steel fiber and alumina fiber.

[0017] Incidentally, although it is usual that the homoacrylic fiberand/or the homoacrylic pulp are/is made into sheet together with theabove-mentioned other fiber component or, in some cases, with a frictionadjusting material or the like and a fibrous base material is preparedas a base paper, it is also possible that the homoacrylic fiber issubjected to a crimping treatment, the resulting crimp wedding of 8 to76 mm is made into a nonwoven fabric by a dry nonwoven fabric togetherwith other fiber component or made into a sheet by a textile by spunyarn and the product is used as a fibrous base material for wetfrictional materials.

[0018] When the fibrous base material is prepared particularly by amethod for producing a base paper by making into sheet, there isproduced a structure where fiber is mostly aligned in a horizontaldirection. However, the wet frictional material prepared from thisfibrous base material works by receiving repeated compression in avertical direction and, therefore, there is a problem that elasticrebound force of fiber hardly works effectively in such a fibrous basematerial where fiber is aligned in a horizontal direction. Under suchcircumstances, as a result of investigation on the problem as such, ithas been found that, when the homoacrylic fiber is subjected to acrimping process to give a curved homoacrylic short fiber where thefiber is curved and further cut in short, the short fiber is in athree-dimensional structure in the fibrous base material and, therefore,an elastic rebound force of the fiber to surface pressure upon slidingeffectively works and, further, a wet frictional material having littledeterioration can be prepared.

[0019] In a fiber which is subjected to a crimping process, the fiberlength is preferably 0.5 to 4 mm which is suitable for achieving thethree-dimensional structure in its base paper. When it is shorter than0.5 mm, detachment of the fiber takes place and, moreover, cost forcutting becomes high and deviation in fiber length by erroneous cuttingbecomes high as well whereby that is not preferred. When it is longerthan 4 mm, it is hardly dispersed in a single fiber in water uponpreparation of a base paper but non-dispersed blocks of the fiber areproduced and there is a problem that the homogeneity of surface of thefibrous base material is deteriorated whereby that is not preferred.With regard to an apparatus for giving the crimps, the conventionalcrimping apparatus for the formation of crimps may be used. Althoughthere is no particular limitation for the degree of crimp, it ispreferred to be 7 to 13% and 6-13/inch in terms of crimping rate Ci andcrimp numbers Cn, respectively, which are usually used in wedding forspinning.

[0020] When a base paper is adopted as a fibrous base material forforming the wet frictional materials, it is particularly effective touse a pulp fiber having a binder effect for improving the strength ofthe base paper. As the strength of the base paper is improved, itsstrength to sliding stress of the wet frictional material also becomeshigh and durability to crack and damage upon working for a long term isalso improved. In addition, the fibrillated pulp fiber effectively worksfor making the above fibrous base material into a three-dimensionalstructure as well and, therefore, it is preferred to use a homoacrylicpulp where the homoacrylic fiber is fibrillated as a material havingsuch pulp characteristics in addition to the characteristics of thehomoacrylic fiber adopted by the present invention.

[0021] The homoacrylic pulp is prepared by fibrillation of theabove-mentioned homoacrylic fiber. Although it is possible to utilize acommon beating machine for the fibrillation, the homoacrylic fiber istough and some means are needed for beating. Thus, when a refiner isused, a dispersion where the homoacrylic fiber is cut in 2 to 6 mm anddispersed solely in a concentration of 1 to 4% in water without otherfiber is to be used and beating is to be carried out at high electriccurrency using a beater such as a single refiner where a high shear isapplied and using a tooth profile for twisting beating. Freeness whichis a yardstick for fibrillation of the homoacrylic pulp can beappropriately adjusted to give pore size and base paper strengthsuitable for the use adjusting to the balance for other fiber componentand friction adjusting material and 260 to 600 ml in terms of CanadianStandard Freeness is preferred in view of the balance between the bindereffect and the cost for fibrillation.

[0022] Even when an acrylic pulp is used solely, it effectively works toa fundamental durability of a wet frictional material using a fibrousbase material comprising the same but, as a result of making into pulpby beating the fiber; its surface area is quite large. Therefore,resistance to chemicals such as acidic decomposition product tends tosomewhat lower as compared with the homoacrylic fiber. Now the joint useof the homoacrylic fiber with the homoacrylic pulp in an appropriateratio is more effective. Thus, when the ratio by weight of thehomoacrylic fiber to the homoacrylic pulp is from 50/50 to 10/90, thereis prepared a good fibrous base material for wet frictional materials inwhich resistance to acid of the fibrous base material and strength ofthe fibrous base material are well balanced.

[0023] When a base paper is used as a fibrous base material for wetfrictional materials according to the present invention, it is preparedin such a manner that fiber components comprising the homoacrylic fiberand/or the homoacrylic pulp and the above-mentioned other heat-resistingfiber, a friction adjusting material and a filler are dispersed togetherin water, the resulting slurry is aggregated by an aggregating agent,made into a sheet using a paper machine and the resulting sheet is driedand subjected to molding by punching. The friction adjusting agent andthe filler may be added by means of coating or impregnation after thepaper making. It is preferred that fiber length of the homoacrylic fiberis 0.5 mm to 4 mm in view of the above-mentioned balance between thecutting cost and dispersibility upon dispersing into water. When a drynonwoven fabric or woven fabric is made into a fibrous base material,nonwoven fabric or woven fabric is previously prepared using the fibercomponents and then a friction adjusting agent and other filler areimpregnated therein or coated thereon. Incidentally, with regard to thefriction adjusting material used here, known ones such as cashew dust,graphite, molybdenum disulfide and aluminum powder may be used. Withregard to the filler, barium sulfate, calcium carbonate, magnesiumcarbonate, diatomaceous earth, etc. may be used.

[0024] The fibrous base material of the present invention is very usefulfor the preparation of a wet frictional material having an excellentdurability for a long period. In the preparation of the wet frictionalmaterial, a binder represented by a thermosetting resin and the fibrousbase material of the present invention are molded by pressure and heataccording to the conventional method and, if necessary, metal fittings,etc. are attached thereto. Representative examples of theabove-mentioned resin are phenol resin, epoxy resin, melamine resin,imide resin and polyester resin.

EXAMPLES

[0025] As hereunder, Examples are shown for making understanding of thepresent invention easy. They are, however, merely exemplary and the gistof the present invention is not limited thereby. Incidentally, part(s)and percentage are those by weight unless otherwise mentioned.

[0026] Preparation of Fiber Samples

[0027] According to the conventional method, an acrylonitrilehomopolymer was dissolved in a solvent, spun, subjected to a wet-heatdrawing (draw ratio: 6 times), subjected to a dry-heat drawing andsubjected a heat setting for 3 minutes under a fiber tension to give thehomoacrylic fibers of No. 1 to No. 4 of the present invention. Table 1shows condition for the manufacture of each homoacrylic fiber, primarymodulus of elasticity at room temperature, modulus of elasticity at 180°C. and shrinking rate at 200° C. The homoacrylic fiber No. 4 of thepresent invention is that prepared by a crimping process and its Cn was10/inch and Ci was 10%. Incidentally, molecular weights of the polymerin the table are weight-average molecular weights. For the sake ofcomparison, there are similarly shown No. 5 which is a homoacrylic fibersatisfying no essential features of the homoacrylic fiber of the presentinvention and No. 6 which is a copolymerized acrylic fiber as well.Here, primary modulus of elasticity at room temperature was measured inaccordance with JIS-L-1095.9.13 (method for the measurement of initialtensile resistance of fiber). TABLE 1 DR Primary Solvent upon ME at SRat for Monomer Spinning Dry Thermosetting Crimping room ME at 200° C.No. Spinning Used Mw (×1000) Method Heating Temp Treatment temp 180° C.(%) 1 PRS 100% AN 200 AG 2.0 215° C. − 160 13 3 2 PRS 100% AN 150 AG 1.6205° C. − 110 11 5 3 DMSO 100% AN 150 AG 1.6 205° C. − 110 10 5 4 PRS100% AN 200 AG 2.0 215° C. + 165 13 4 5 PRS 100% AN 120 Wet 1.6 170° C.− 90 3 15 method 6 PRS 3% VAc 150 AG 2.0 205° C. − 150 7 5 and 97% AN

[0028] Preparation of Homoacrylic Pulp

[0029] The above homoacrylic fiber No. 1 was cut into 6 mm and aresulting single aqueous slurry of 2% concentration was fibrillated by arefiner (details: Superfibrator 600A manufactured by Hasegawa Tekko wasequipped with profile of type 611S and used to beat with electriccurrent of 80 Amp at 440 volts; adjustment of the freeness was conductedby adjusting the passing numbers of the refiner) to give a homoacrylicpulp as shown in Table 2. Freeness was measured according to JIS-P-8121in accordance with the Canadian Standard Freeness. For comparison,designed freeness (catalog data) is also shown for No.3 and No.4 whichare CFFV 125 and CFFV 110, respectively, manufactured by Sterling whichare acrylic pulps from a copolymerized acrylic fiber. TABLE 2 No.Acrylic Fiber Used Freeness (CSF) 1 Homoacrylic fiber No. 1 in Table 1600 ml 2 (same as above) 260 ml 3 (Commercially available acrylic pulp)400 ml 4 (same as above) 250 ml

[0030] After that, 60 parts of the homoacrylic fiber together with thehomoacrylic pulp and other fiber components, 8 parts of cashew dust and7 parts of graphite as friction adjusting materials and 15 parts ofdiatomaceous earth as a filler were mixed and milled using a hand papermachine to prepare a fibrous base material of 1 mm thickness for wetfrictional materials. Here, with regard to the other fiber components,linter pulp/aramid pulp/potassium titanate fiber/carbon fiber in a ratioof 50/40/5/5 by weight were used. Table 3 shows Nos. of the homoacrylicfiber (A) and the homoacrylic pulp (B) used for the fibrous basematerial, compounding ratio of A and B to the total fiber components andratio of A to B. For comparison, acrylic fiber and acrylic pulp whichare out of the scope of the present invention are also shown similarly.

[0031] The fibrous base material was cut into 2 cm width and 20 cmlength and the resulting test piece was subjected to a measurementaccording to JIS-L-1096-A where grasping length was 15 cm (in alongitudinal direction) and tensile speed was 10 cm/minute using aTensilon measuring machine and the result was adopted as the strength ofthe fibrous base material. Further, with regard to the test forresistance to acid, a test piece in the same size was dipped in 35%diluted sulfuric acid, pulled out, air-dried as it was and thensubjected to repeated compression and release every 5 seconds using apress molding machine (type M-2 manufactured by Shinto) at hot platetemperature of 180° C. and surface pressure of 100 kg/cm². Afterrepeating for 100,000 times, the same measurement as that in theabove-mentioned strength for the fibrous base material was carried outand percentage to the strength of the fibrous base material in theinitial stage (before the test for resistance to acid) was adopted as aretention rate of strength after the test for resistance to acid. At thesame time, thickness of the test piece was measured and a reduction fromthe thickness of the fibrous base material in the initial stage wasadopted as the deteriorated amount after the test for resistance toacid. Incidentally, measurement of thickness was conducted at randomfive places of the test piece followed by averaging. With regard toevaluation of the shrinking rate upon heating, a test piece cut into asize of 20 cm width and 20 cm length was heated for 10 minutes by ahot-air drier of 200° C. and subjected to natural cooling, longitudinaland vertical sizes were measured and shrunk area to the initial area wasexpressed in terms of percent. Table 3 shows strength, retention rate ofstrength after the test for resistance to acid and shrinking rate uponheating of the fibrous base material. TABLE 3 Rate of Shrinking AcrylicFiber A Acrylic (A + B) After Acid-Resisting Test Rate of Cut Pulp B inTotal Strength Retention Rate Deteriorated FBM upon No. in length (No.in Fiber of FBM of Strength Amount Heating No. Table 1 (mm) Table 2) A/BComponent (kgf) (%) (%) (%) Ex. 1 1 1 1 30/70 30 0.82 45 10 6 2 2 1 130/70 30 0.81 43 12 6 3 1 1 1 10/90 30 1.05 44 13 6 4 — — 1  0/100  51.02 37 19 6 5 2 1 — 100/0  50 0.65 34 14 10 6 4 1 2 30/70 30 0.87 50 66 7 4 4 2 30/70 30 0.89 51 7 6 8 4 0.5 2 30/70 30 0.85 49 7 6 9 3 1 150/50 30 0.81 40 18 6 CE 10 5 1 3 30/70 30 0.80 18 30 28 11 6 1 4 30/7030 0.82 * 12

[0032] In the fibrous base materials (No. 1 to No. 9 in Table 3) inwhich the homoacrylic fiber (No. 1 to No. 4 in Table 1) and thehomoacrylic pulp (No. 1 and No. 2 in Table 2) meeting the requirementsof the present invention were used in 5% to 50% of the fiber components,shrinkage upon heating was small and both retention rate of strength anddeteriorated amount after the test for resistance to acid showed gooddata. The above constitutes a base for the facts that there is littleproblem in adhesion of core metal upon preparation of a wet frictionalmaterial and that, when used as a wet frictional material, durabilitywas high and stable friction characteristic was achieved. On thecontrary, in the product (Comparative Example No.10 in Table 3) wherethe homoacrylic fiber having high shrinkage upon heating at 200° C. (No.5 in Table 1) was used, shrinking rate of a fibrous base material uponheating was high and there is estimated a problem of detachment from acore metal by shrinking upon molding of a wet frictional material and,in the product (Comparative Example No. 11 in Table 3) where the acrylicfiber (No. 6 in Table 1) which is a copolymer was used, there was founda problem in durability that deterioration in the test for resistance toacid was big and the test piece was broken during the test.

[0033] As compared with Examples No. 1 to No. 3 and No. 6 to No. 9 wherethe homoacrylic fiber and the homoacrylic pulp were used together,Example No. 4 where only homoacrylic pulp was used showed a bit of a bigdeteriorated amount after the test for resistance to acid while, inExample No. 5 where only homoacrylic fiber was used, strength of thefibrous base material and retention rate of strength after the test forresistance to acid considerably lowered whereupon it is noted that thejoint use of the homoacrylic fiber and the homoacrylic pulp ispreferred. In Example 1 where the homoacrylic fiber (No. 1 in Table 1)having a preferred modulus of elasticity at 180° C. was used,deteriorated amount after the test for resistance to acid was far lessresulting in a good data as compared with Example 2 but, in Examples No.6 to No. 8 where a curved homoacrylic short fiber having preferredmodulus of elasticity at 180° C., deteriorated amount was still lowerand retention rate of strength was also high showing an excellentdurability whereupon it is noted that there are prepared excellentfibrous base materials for wet frictional materials showing a goodbalance to strength of the fibrous base material.

[0034] In Example No. 9, there was used the homoacrylic fiber which wasspun using DMSO which is an organic solvent. As compared with ExampleNo. 2 (where there was used the homoacrylic fiber which was spun usingPRS which is an inorganic aqueous solvent) having the same fibrous basicmaterial composition except the homoacrylic fiber, there was noted atendency that deteriorated amount after the test for resistance to acidincreased and retention rate of strength also lowered. Incidentally, inthe Examples, strength of fibrous base material of No. 4 is high andthat will be due to the fact that pulp components such as linter pulpand aramid pulp increased as a whole in addition to the homoacrylicpulp.

1. A fibrous base material for wet frictional materials which comprisesa fiber component, a friction adjusting material and a filler, saidfiber component containing 5 to 50% by weight of a homoacrylonitrilefiber where modulus of elasticity at 180° C. is not less than 10 cn/dtexand shrinking rate at 200° C. is within 5% and/or a homoacrylonitrilefiber in a pulp form prepared by fibrillation of said homoacrylonitrilefiber:
 2. A fibrous base material for wet frictional materials accordingto claim 1, wherein ratio by weight of the homoacrylonitrile fiber tothe homoacrylonitrile fiber in a pulp form is from 50/50 to 10/90.
 3. Afibrous base material for wet frictional materials according to claim 1,wherein the homoacrylonitrile fiber is a fiber which is subjected to acrimping treatment followed by cutting into 0.5 to 4 mm.
 4. A fibrousbase material for wet frictional materials according to claim 2, whereinthe homoacrylonitrile fiber is a fiber which is subjected to a crimpingtreatment followed by cutting into 0.5 to 4 mm.